Category Archives: Science Lessons

Snowstorms and Sea Ice

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Cross-posted from NextGen Journal

“That’s some global warming”, Fox News proudly announced. “Rare winter storm dumps several inches of snow across South.” It’s cold outside, and/or it’s snowing, so therefore global warming can’t be happening. Impeccable logic, or rampant misconception?

It happened last winter, and again so far this season: unusual snow and extreme cold thrashed the United States, Europe, and Russia. Climate change deniers, with a response as predictable as Newton’s Laws, trumpeted the conditions as undeniable proof that the world simply could not be warming. Even average people, understandably confused by conflicting media reports, started to wonder if global warming was really such a watertight theory.

But sit and think about it for a minute. If it’s cold right now in the place where you live, that doesn’t mean it’s cold everywhere else. It’s simply not possible to look at your little corner of the world and extrapolate those conditions to the entire planet. There’s a reason it’s called global warming, and not “everywhere-all-the-time warming”. Climate change increases the amount of thermal energy on our planet, but that doesn’t mean the extra energy will be distributed equally.

That said, an interesting weather condition has been prominent over the past month, telling a fascinating story that begins in the Arctic. At the recent American Geophysical Union conference in San Fransisco, the largest annual gathering of geoscientists in the world, NOAA scientist Jim Overland described the situation.

Usually in winter, the air masses above the Arctic have low pressure, and the entire area is surrounded by a circular vortex of wind currents, keeping the frigid polar air contained. Everything is what you’d expect: a cold Arctic and mild continents. These conditions are known as the positive phase of the North Atlantic Oscillation (NAO), an index of fluctuating wind and temperature patterns that impacts weather on both sides of the Atlantic.

The negative phase is different, and quite rare: high pressure over the Arctic forces the cold air to spill out over North America and Eurasia, allowing warm air to rush in to the polar region. Meteorologist Jeff Masters has a great analogy for a negative NAO: it’s “kind of like leaving the refrigerator door ajar–the refrigerator warms up, but all of the cold air spills out into the house.” The Arctic becomes unusually warm, and the temperate regions of the surrounding continents become unusually cold. Nobody visually depicts this pattern better than freelance journalist Peter Sinclair:

So what’s been causing this rare shift to the negative NAO the past two winters? In fact, global warming itself could easily be the culprit. Strong warming over the Arctic is melting the sea ice, not just in the summer, but year-round. Open water in the Arctic Ocean during the winter allows heat to flow from the ocean to the atmosphere, creating the high pressure needed for a negative NAO to materialize. Paradoxically, the cold, snowy weather many of us are experiencing could be the result of a warming planet.

An emerging debate among scientists questions which force will win out over winters in Europe and North America: the cooling influence of more negative NAO conditions, or the warming influence of climate change itself? A recent study in the Journal of Geophysical Research predicts a threefold increase in the likelihood of cold winters over “large areas including Europe” as global warming develops. On the other hand, scientists at GISS, the climate change team at NASA, counter that extreme lows in sea ice over the past decade have not always led to cold winters in Europe, as 7 out the past 10 winters there have been warmer than average.

Amid this new frontier in climate science, one thing is virtually certain: global warming has not stopped, despite what Fox News tells you. In fact, despite localized record cold, 2010 is expected to be either the warmest year on record or tied for first with 2005 (final analysis is not yet complete). What you see in your backyard isn’t always a representative sample.

Does Breathing Contribute to CO2 Buildup in the Atmosphere?

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I was recently honoured to join Skeptical Science, a comprehensive database of rebuttals to common climate change misconceptions, as an author. Here I am republishing my first article regarding the common myth that breathing out contributes to the buildup of atmospheric carbon dioxide. It is the Intermediate version, and I have also written a very similar Basic version, which includes a diagram by John Cook. Enjoy!

The very first time you learned about carbon dioxide was probably in grade school: We breathe in oxygen and breathe out carbon dioxide. Any eight-year-old can rattle off this fact.

More specifically, the mitochondria within our cells perform cellular respiration: they burn carbohydrates (in the example shown below, glucose) in the oxygen that we breathe in to yield carbon dioxide and water, which we exhale as waste products, as well as energy, which is required to maintain our bodily processes and keep us alive.

C6H12O6 + 6O2 → 6CO2 + 6H2O + energy

carbohydrates + oxygen → carbon dixoide + water + energy

It should come as no surprise that, when confronted with the challenge of reducing our carbon emissions from the burning of fossil fuels, some people angrily proclaim, “Why should we bother? Even breathing out creates carbon emissions!”

This statement fails to take into account the other half of the carbon cycle. As you also learned in grade school, plants are the opposite to animals in this respect: Through photosynthesis, they take in carbon dioxide and release oxygen, in a chemical equation opposite to the one above. (They also perform some respiration, because they need to eat as well, but it is outweighed by the photosynthesis.) The carbon they collect from the CO2 in the air forms their tissues – roots, stems, leaves, and fruit.

These tissues form the base of the food chain, as they are eaten by animals, which are eaten by other animals, and so on. As humans, we are part of this food chain. All the carbon in our body comes either directly or indirectly from plants, which took it out of the air only recently.

Therefore, when we breathe out, all the carbon dioxide we exhale has already been accounted for. By performing cellular respiration, we are simply returning to the air the same carbon that was there to begin with. Remember, it’s a carbon cycle, not a straight line – and a good thing, too!

A Fabulous Contribution

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I’ve really been enjoying the Advanced versions of Skeptical Science’s rebuttals to common misconceptions about climate change. So far, they have all been written by someone going by the name of dana1981, who I would like to give a huge shout-out to. I am a new B.Sc. student who is interested in pursuing a career in climate change research, and these articles have been very helpful in giving me a taste of basic atmospheric science.

In “How do we know more CO2 is causing warming?”, I was introduced to the relatively simple equation required to calculate the radiative forcing of increased atmospheric carbon dioxide, as well as the expected equilibrium temperature change from CO2, using the range of values for climate sensitivity provided by the IPCC (as calculating climate sensitivity is not quite so simple!)

In “The human fingerprint in global warming”, dana1981 discussed different attribution studies, and explained how anthropogenic warming has certain “fingerprints” – more warming at night than during the day, a cooling of the stratosphere, and a rise in tropopause height – all of which have been observed. I had a basic understanding of these fingerprints and why they occurred, but it was great to read about the current research in attribution studies, with impeccable citations.

“How sensitive is our climate?” was similar to the first article, but also addressed the common misconception that climate sensitivity is specific to different forcings. If the climate has low sensitivity to CO2, it also has low sensitivity to solar radiation, cosmic ray feedback, etc. The equilibrium temperature change doesn’t care if the extra few W/m2 is from the greenhouse effect or planetary albedo – it changes with the same speed either way, which disproves many skeptical arguments. Additionally, since the prehistoric record shows large swings in climate resulting from relatively small forcings, scientists are confident that climate sensitivity is not very low.

“Solar activity & climate: is the sun causing global warming?” was absolutely fascinating. The equations required to calculate solar forcing using total solar irradiance were new to me, and dana1981 went so far as to analyze early 20th-century warming, calculating how much was due to an upswing in solar irradiance and how much was due to anthropogenic greenhouse gases. During the latter half of the 20th century, solar irradiance has dropped back down, but warming has only accelerated.

Skeptical Science’s recent efforts to expand their rebuttals to include beginner, intermediate, and advanced levels of explanation were inspired by a RealClimate post written by Dr. Gavin Schimdt. He thoughtfully wrote,

I think we should be explicitly thinking about information levels and explicitly catering to different audiences with different needs and capabilities. One metaphor that might work well is that of an alpine ski hill. There we have (in the US for instance) green runs for beginners wanting a gentle introduction and where hopefully nothing too bad can happen. Blue runs where the technical level is a little more ambitious and a little more care needs to be taken. Black expert runs for those who know what they are doing and are doing it well, and finally, double black diamond runs for the true masters. No-one accuses ski resorts of being patronising when they have green runs interspersed with the more difficult ones, and neither do they get accused of elitism when one peak has only black runs going down (as I recall all too painfully on my first ski outing). People self-segregate and generally find their way to the level at which the feel comfortable – whether they want a easy or challenging ride – and there is nothing stopping them varying the levels as their mood or inclination takes them.

Skeptical Science took up this challenge, and although their efforts have largely been focused on creating “plain-English” beginner articles, as a huge target audience for climate change communication is the general public, I’m extremely grateful that they’re also catering to new science enthusiasts such as myself with the advanced articles. Please, keep them coming!

While we’re on the topic, I should also mention a great new post by Skeptical Science, which is not part of their argument database – “The contradictory nature of global warming skepticism”. You can’t hold the objection that the world isn’t warming and then turn around and say that global warming is natural, but these and other self-disproving arguments reach us on a daily basis. Deniers can’t seem to agree on a single unified objection to anthropogenic global climate change, and some individuals, as the post shows, contradict themselves up to five times in six months.

And hey, I just realized right now – that post was also written by dana1981. Whoever this writer is, he or she is doing a great job.

Forcings

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Last time, we talked about the energy budget – the process of radiation coming in from the sun, being absorbed by the Earth, and then emitted as infrared radiation, which we perceive as heat when it hits us. Remember that this outgoing emission of energy is what determines the temperature of the Earth.

So how can the temperature of the Earth be changed? Naturally, there is a lot of year-to-year variation. For example, when the oceans absorb radiation from the sun, they don’t always emit it right away. They will store energy for a long time, and sometimes release lots at once, during El Nino. This kind of internal variability makes the average global temperature very zig-zaggy.

We need to revise the question, then. The question is not about the average global surface temperature – it’s about the amount of energy on the planet. That’s generally how the climate is changed, by increasing or decreasing the amount of energy the Earth emits as infrared radiation, and consequently, the temperature.

There are two ways to do this. The simplest method is to change the amount of incoming energy. By increasing or decreasing the amount of solar radiation that hits the Earth – either directly, by changing the sun’s output, or indirectly, by increasing the albedo or reflectivity of the Earth – the amount of infrared radiation emitted by the surface will also increase or decrease, because incoming has to be equal to outgoing. The change in outgoing radiation will often take a bit of time to catch up to the change in incoming radiation. Until the two reach a new equilibrium, the Earth will warm or cool.

Another way to change the Earth’s temperature is by artificially changing the amount of incoming energy. The same amount of solar radiation reaches the Earth, but when it is absorbed and emitted, some of the emitted infrared energy gets bounced back so the Earth has to absorb and emit it again. By processing the same energy multiple times, the temperature is a lot warmer that it would be without any bouncing. We refer to this bouncing as the “greenhouse effect”, even though greenhouses work in a completely different way, and we will be discussing it a lot more later. By increasing or decreasing the greenhouse effect, the temperature of the Earth will change too.

A change in incoming energy is referred to as a radiative forcing, because it “forces” the Earth’s temperature in a certain way, by a certain amount. It is measured in watts per square meter (W/m2), and it doesn’t take very many watts per square meter to make a big difference in the Earth’s temperature. The resulting change in temperature is called a response.

My favourite analogy to explain forcing and response uses one of the most basic physics equations – F=ma. Mass (m) is constant, so force (F) is proportional to acceleration (a). Applying a forcing to the Earth is just like pushing on a box. If the force is big enough to overcome friction, you get an acceleration – a response.

It’s also very important to use net force, not just any force. If there are two people pushing on the box in different directions with different amounts of force, the acceleration you observe will be equal to the result of those forces combined. Similarly, there are often multiple forcings acting on the climate at once. The sun might be getting slightly dimmer, the albedo might be decreasing, the greenhouse effect might be on the rise. The response of the climate will not match up to any one of those, but the sum of them all together.

Here is a video I made last year, in collaboration with Climate Change Connection, about this very analogy:

In future posts, I will be discussing different forcings in more detail. Stay tuned!

Snowball Earth

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Of all the books I have read about climate change, Snowball Earth, by Gabrielle Walker, is definitely one of the best – and it’s not even about the current climate change.

Part of what makes it so good is the style of writing. As the Los Angeles Times said about her later book, An Ocean of Air, “Walker has a Ph.D. in chemistry, but she writes like a poet.” And, indeed, after an education at Cambridge, Walker has spent most of her career as a science journalist. It’s sort of sad that this doesn’t happen more often. Usually, those who understand a subject best are not the ones who communicate it. Walker is the exception to this rule.

Take, for example, this passage about the history of life on Earth:

Stretch your arms out wide to encompass all the time on Earth. Let’s say that time runs from left to right, so Earth was born at the tip of the middle finger on your left hand. Slime arose just before your left elbow and ruled for the remaining length of your left arm, across to the right, past your right shoulder, your right elbow, on down your forearm, and eventually ceded somewhere around your right wrist. For sheer Earth-gripping longevity, nothing else comes close. The dinosaurs reigned for barely a finger’s length. And a judicious swipe of a nail file on the middle finger of your right hand would wipe out the whole of human history.

Another impressive aspect of Walker’s writing is her characterization. Wacky, stubborn, and exuberant scientists are brought to life. Instead of just hearing about their work and accomplishments, you feel like you’re getting to know them as people. She writes about arguing scientists particularly well. Arguing scientists are so much fun to read about – that’s one reason I loved The Lost World by Arthur Conan Doyle.

However, the best part of this book, by far, is the subject matter. The theory of Snowball Earth is possibly the most awesome thing I have ever heard about. Here’s how the story goes:

From what paleontologists can see preserved in fossils, complex life arose at a very specific point in prehistory: the end of the Precambrian. For several billion years before that, the only thing that lived on Earth was unicellular goop. But then, suddenly, all at once, complex organisms burst onto the evolutionary stage.

Something must have caused this dramatic appearance, and a series of scientists from the 1940s on – most prominently, Paul Hoffman – likely have discovered what. At the end of the Precambrian, there are signs of ice in rocks all over the world – scratches, rock deposits, everything that led Agassiz to discover the ice ages.

Because plate tectonics moves everything around so much, though, rocks were not necessarily formed at the location they sit today. Their magnetic field is what discloses their birthplace. Tiny bits of magnetic material, such as iron, line their field up with the Earth’s. The Earth’s magnetic field is perpendicular to the surface at the poles and parallel to the surface at the Equator, like this:

So, if a rock’s magnetic field is vertical, it was formed at the poles. If it is horizontal, it was formed at the equator. Incredibly, scientists found Precambrian rocks, with signs of ice, with horizontal magnetic fields. During that period of prehistory, the equator was covered in ice – and, therefore, the whole planet, because it’s not really possible to freeze the equator without freezing all the other latitudes too.

The scientists determined that, for several instances on the Precambrian, the continents were arranged in a way that was very conducive to ice-albedo feedback. With the smallest trigger, ice from the poles would creep across the temperature zones and meet at the equator. Frozen oceans, frozen land, the whole bit.

And now CO2 comes into the story. Volcanic eruptions naturally release carbon dioxide, but the amount is so small that the oceans have no trouble soaking them up – unless they’re frozen on the surface and cut off from the air. CO2 would gradually build up, in that case, and millions of years later, the greenhouse effect would be so strong that all the ice would melt and the planet would plunge into a state referred to as Hothouse Earth. Then the oceans would start absorbing all the extra CO2, and ice would reappear at the poles, and the cycle would begin again.

Many scientists believe that these Precambrian cycles of extreme heat and extreme cold provided such a strong pressure on organisms that natural selection was pushed to new boundaries. Complex life had an advantage in these extreme conditions, and it flourished. The most catastrophic climatic event our planet has ever experienced, in our knowledge, was what led to the evolution of multicellular organisms, and eventually, us.

It makes me feel very small, the same way that attempting to comprehend the vastness of the universe makes me feel very small. The life we see all around us only exists because of a series of coincidences. Human beings, one of the youngest of the millions of animal species that have ever existed, are alive because of continental drift lining things up in the right way. And who knows what would have happened if things had been slightly different?

The Energy Budget

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I’ve decided to take this blog more in the direction of communicating science – there are only so many sociological musings to come up with. This is the first of many planned posts explaining basic climate science so people have better context for what they read in the newspaper.

Every post is a work in progress, and will be continuously edited when necessary, so please leave comments with suggestions on how to improve the accuracy or clarity. Enjoy!

What determines the temperature of the Earth?

The temperature in your backyard, the warmth of the equator, the frigid polar regions, the average global temperature for the whole planet…..they might seem like very different things to measure, but they’re all caused by the same process. It all comes back to energy.

This energy comes from the Sun, but it’s not as simple as a single transfer. Remember, at any time of the day or night, the Sun is shining on some part of the Earth. That energy can’t just stay on our planet, otherwise it would keep building up and up and we would fry after a couple of weeks.

Therefore, incoming energy from the Sun has to be balanced by outgoing energy from the Earth for the planet’s temperature to stay relatively constant. So when the Sun’s rays hit the ground, as a mixture of light, infrared, and UV radiation, the Earth absorbs the energy. Then it converts it to all to infrared radiation, which we perceive as heat when it hits us, and releases it upward.

All objects perform this absorption and emission when they are hit with radiation. If they receive enough energy, they can release some of it in the form of light – think of how a stove element glows when it’s turned on. However, the energy hitting the Earth is nowhere near this level, so it all comes out as infrared.

It is this emission of infrared radiation that determines the temperature of the Earth. The second step, not the first, is the important one, the one that we actually feel and experience. So on a hot summer’s day, it isn’t actually energy coming down from the Sun that’s making the air warm. It’s energy coming up from the Earth.

The air doesn’t warm up instantly, either – there’s a bit of a lag. This allows warm air to be transported away from the Equator and towards the poles, in the global circulation system of wind currents. Without this lag time, many regions of our world would have far more extreme temperatures.

Additionally, not all the radiation the Sun sends down gets absorbed by the Earth. Some of it is bounced back by clouds, which is why sunny days tend to be warmer than cloudy days. Some of it reaches the surface of the planet, but is bounced back too, before it’s even absorbed. This reflection of energy is particularly common when the surface is light in colour. That’s why it seems so bright outside after a snowstorm – because the snow is bouncing the energy back up as light, instead of absorbing it and releasing it upward as heat. It also explains why dark concrete, which absorbs almost all the radiation that hits it, is so much warmer than a light-coloured deck.

The amount of energy that the Sun sends down to us is greater than the amount that the surface of the Earth actually absorbs. However, the amount absorbed has to be equal to the amount released, and the amount released is what we witness as the temperature outside.

We Have Slides!

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After a marathon PowerPoint-session yesterday I finally got my 63 slides out of the way. Here is the presentation for anyone who is interested. The script is written in the notes beneath the slides.

I like to have things fading in and out of my slides, so sometimes the text boxes and images are stacked on top of each other and it won’t make sense until you view the animation.

Researching the median lethal dose of arsenic during my spare at school was really awkward. I had to do a lot of hasty explaining to my friends about how it was a metaphor for small concentrations having large effects, and no, I wasn’t planning to poison anyone.

Anyway, enjoy.

Mind the Gap (12 MB)

How to Prove Global Warming Wrong

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Over the past twenty years, vested interests and political lobby groups have done a fantastic job confusing the public about anthropogenic climate change. To many, they seem to have proven the whole theory wrong.

But how could you actually prove global warming wrong – not just in the minds of the public, but through the established scientific process? What scientific discoveries – if they held up through peer-review, further criticism, and replication – would render climate change a non-problem?

One of the surest ways to stop all this cap-and-trade discussion would be to disprove the greenhouse effect itself – the mechanism by which the Earth absorbs and emits the same energy multiple times, due to the presence of greenhouse gas molecules that “bounce it back”. This keeps the Earth substantially warmer than it would be otherwise. Additionally, if the concentrations of greenhouse gases increase, so will the temperature of the Earth. This process was first hypothesized by Joseph Fourier in 1824, and was experimentally confirmed by John Tyndall in 1856. The first prediction of eventual man-made global warming came from Svante Arrhenius, in 1896. It wasn’t a theory as much as a logical result of a theory, one that was deeply rooted in physics and chemistry.

Unless our understanding of entire fields of physical science is totally off base, we can be sure that our greenhouse gas emissions will cause climate change eventually. But hey, if you could overturn all of thermodynamics, you wouldn’t have to worry about carbon taxes.

  • Cheap-out option: Svante Arrhenius was Swedish, but his name sounds sort of Russian, and 1896 wasn’t very long before the Russian Revolution. Therefore, Arrhenius was a Communist, and none of his scientific work can be trusted.

Knowing that something is sure to happen eventually, though, is different from knowing that it is happening right now with substantial speed. We know that the Earth is warming – even if you found some statistical way to disprove three separate temperature records, the physical and biological systems of our planet still stand: 90% of observed changes in the natural world, like the blooming of flowers, the peak flows of rivers, and the spawning of fish, are in the direction expected with warming (Rosenzweig et al, 2008).

But how do we know that the warming is caused by us? Climate change has been caused many times in the past by factors unrelated to greenhouse gases – like solar influences, whether they’re direct (a change in solar output) or indirect (a change in the Earth’s orbit). How do we know that’s not happening now?

If the warming was caused by the sun, the atmosphere would warm uniformly at all levels. However, if the Earth was warming from greenhouse gases, the troposphere (the layer of the atmosphere closest to the planet) would warm while the stratosphere (the next level up) would cool. This is because more heat is getting bounced back to the surface by greenhouse gases, and is subsequently prevented from reaching the stratosphere.

A cooling stratosphere has been described as the “fingerprint” evidence of greenhouse-induced warming. And, in fact, the stratosphere has been cooling over the past 30 years (Randel et al, 2009). Therefore, if you could somehow show that something else was causing this pattern of a warming troposphere and a cooling stratosphere, and that the significant, anthropogenic rise in greenhouse gases was somehow not affecting it, you would have a case for global warming being natural.

Update (18/2/10): About half of this cooling can be attributed to ozone depletion, and the other half can be attributed to greenhouse gases (NOAA, 2006). The flat trend in stratospheric temperatures from 1995-2005 (see the Randel citation above) can be explained by the recovery of ozone, which is temporarily offsetting the greenhouse gases. Interesting how the temperature of the stratosphere has just as many factors as the temperature of the troposphere…..but in both cases, you can’t explain the temperature trends without including human activity. Scott Mandia has a great explanation here.

  • Cheap-out option: Omit the explanation of why greenhouse warming causes stratospheric cooling. Just point to the graph that goes down and say, “The atmosphere is cooling! Therefore, the IPCC is a hoax!”

Finally, even if you couldn’t disprove that global warming is expected, observed, and anthropogenic, you could still show that it isn’t very significant. The way to do this would be to show that climate sensitivity is less than 2 C. Climate sensitivity refers to the amount of warming that would result from a doubling of carbon dioxide equivalent, and 2 C is generally accepted as the maximum amount of warming that our society could endure without too much trouble. The current estimates for climate sensitivity, in contrast, average around 3 C (a range of 2-4.5), and it is very unlikely to be less than 1.5 C (IPCC AR4).

However, a climate sensitivity of less than 2 C only means that climate change isn’t a problem if our greenhouse gases stop at a doubling of carbon dioxide equivalent from pre-industrial levels. Even without taking methane and other greenhouse gases into account, this brings us to a CO2 concentration of 560 ppm, which we are well on track to surpass, even with cap-and-trade. So you’d have to argue for a climate sensitivity of even less. Seeing as we’ve already warmed 0.8 C, it doesn’t leave you with a lot of wiggle room.

  • Cheap-out option: Build a climate model that does what you want it to, without any regard for the laws of physics. ExxonMobil will probably sponsor the supercomputers. Widely publicize the results and avoid peer-review at all costs.

Daunting tasks, certainly. But if you really believe that global warming is natural/nonexistent/a global conspiracy, this is the way to prove it. If you managed to prove it, and change the collective mind of the scientific community (not just the public), you’d probably win a Nobel Prize. So it’s certainly worth your time and effort.

The Greenhouse Effect

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It always helps to have some background scientific knowledge on climate change – it makes it easier to sort credibility and call people’s bluffs. I thought I’d give a brief explanation of the Earth’s energy balance, something that confused me for a long time.

All substances can absorb a certain amount of radiation – they must then emit or radiate it back out, usually in the form of long-wave radiation (heat). Some molecules, however, possess certain chemical properties which allow them to absorb (and therefore emit) an extremely large amount of radiation relative to their size. These molecules are called greenhouse gases. I recently asked a chem major exactly which properties determined this amount of absorption. They replied, “You don’t know enough quantum chemistry yet.”

When solar radiation, in the form of short-wave radiation (light), approaches our atmosphere, about 30% is reflected right off. The remaining 70% reaches the atmosphere and the Earth’s surface.

The surface of the Earth absorbs some of the radiation. It can’t hold onto this energy indefinitely (it wants to have room to absorb the next rays of light), so it emits it back out. Even though it received the radiation in the form of light, it emits it in the form of heat. It is this emission that determines the temperature of the Earth.

Greenhouse gases allow short-wave light to pass straight through the atmosphere, but they do absorb some of the long-wave heat that the Earth just emitted. The more greenhouse gases present, the more radiation the atmosphere, as a whole, can absorb. When atmospheric particles (greenhouse gases, in this case) emit this radiation back out, it goes uniformly in all directions. Some goes up and escapes out to space. But some goes down and hits the Earth’s surface again.

Therefore, the Earth has to absorb and emit some of the radiation twice. This increases the temperature of the Earth.